Field of the Invention
This invention relates to implants used at bone fracture sites and, more particularly, to an implant that is directed into an intramedullary cavity/canal in the fractured bone.
Background Art
As seen in FIGS. 1 and 2, the human elbow joint at 10 is essentially a hinge joint, formed from the articulation of the lower portion of the distal humerus 12 with the proximal portions of the two bones of the forearm—the radius 14 and ulna 16. Where these three bones come in contact with each other, the surface of the bone is covered with articular cartilage which provides a slippery joint lining that allows gliding and motion of the bone on one side of the joint against the bone on the other side of the joint. Anatomically, the articular surface of the distal humerus 12 is formed into two condyles that act like curved runners to allow tracking of the articular surface of the corresponding proximal end of each forearm bone 14, 16. The medial condyle of the distal humerus 12 articulates with the olecranon (the proximal articulating surface of the ulna 16) and the lateral condyle of the distal humerus articulates with the radial head (the proximal articulating surface of the radius 14). Because of this anatomical arrangement, motion between the humerus 12 and the proximal portion of the two forearm bones 14, 16 is limited to simple flexion and extension.
The medial condyle is trochlear or hourglass in shape and is called the trochlea. The trochlea conforms to the C-shaped structure of the olecranon (proximal ulna) and allows simple flexion and extension. The anterior end of the C-shaped proximal ulna at 18 is called the coronoid process which recesses into a corresponding depression on the anterior surface of the distal humerus at 20, called the coronoid fossa, with extremes of elbow flexion. The posterior end of the C-shaped proximal ulna at 22 is called the olecranon process. It recesses into a corresponding depression at 24 on the posterior surface of the distal humerus 12 called the olecranon fossa with extension. Because the coronoid fossa 20 and the olecranon fossa 24 are diametrically positioned on the anterior and posterior surfaces of the distal humerus 12 directly proximal to the articular surface, this central triangular portion of the bone can be quite thin. Occasionally, this portion of the bone is actually absent.
In contrast to the medial condyle at 26, the lateral condyle at 28 is basically spherical in shape and called the capitellum. It conforms with the cup-shaped end of the radial head (proximal radius) 30 and allows gliding of the radial head 30 over the capitellum 28 during simple flexion and extension of the elbow. In addition, it also allows the radial head 30 to rotate on the capitellum 28 with movement of the forearm into pronation and supination.
Proximal to the articular condyles of the distal humerus 12, the distal end of the humerus has bony prominences on both the medial and lateral aspects of the bone. These prominences are called the medial epicondyle 32 and the lateral epicondyle 34, respectively. Each of these epicondyles functions as an anchor point for attachment of the strong muscles of the forearm, with the strong flexor and pronator group of muscles attached to the medial epicondyle and the strong extensor and supinator group of muscles attached to the lateral epicondyle 34. Because of the combination of the bony pillars that make up the medial and lateral sides of the distal humerus 12 with the thin central area formed from the olecranon and coronoid fossas, the distal humerus 12 structurally is essentially triangular, with medial and lateral columns of bone that are connected distally with a horizontal osseous pillar made up by the combination of the capitellum 28 and trochlea 36.
Fractures of the distal humerus 12 can be simple or complicated. Reference is made to FIGS. 3-8 which show different types of fractures, successively at F1, F2, F3, F4, F5, F6. Supracondylar fractures describe fractures that extend across the bone with a fracture line that typically crosses the region of the thin olecranon and coronoid fossas. Supracondylar fractures may be simple with a single transverse fracture line, comminuted with intermediate segments that extend up the shaft, or involve fragmentation of the articular surface, such as T-condylar fractures, as seen in FIGS. 7 and 8. In contrast, the term condylar fractures describe fractures that involve only the medial or lateral condyle. Lateral condylar fractures are shown in FIGS. 3 and 4 with medial condylar fractures shown in FIGS. 5 and 6.
Treatment of condylar and supracondylar fractures can be challenging. Because large bending forces are generated by the long lever arms of the humerus and forearm, closed methods of treatment such as simple cast immobilization often are ineffective. Interfragmentary pins 38, as seen in FIGS. 9a-9d, have been used to supplement fixation, but this fixation is often tenuous and poses difficulties to obtain and hold an accurate reduction (restoration of the joint anatomy). These pins 38 are shown in FIGS. 9a-9d on bone parts produced by a supracondylar fracture. In addition, since pin fixation lacks structural rigidity, treatment typically requires prolonged immobilization and can often result in permanent stiffness and dysfunction of the joint.
In an effort to overcome these problems, open reduction and internal fixation have been used in an attempt to achieve anatomic restoration of the joint that is rigid enough to allow early motion. Typically, open reduction internal fixation uses standard pins, screws and plates or combinations of these components. In addition to the objective of restoration of joint anatomy, open reduction internal fixation should avoid further morbidity and complications from the internal fixation itself. Unfortunately, existing methods of internal fixation often fall far short of achieving these goals.
As seen in FIG. 10, use of medial plates MP, lateral plates LP, and screws S is the most common form of internal fixation. With this method, the fracture is reduced and temporarily held in place while plates LP, MP are applied to the bone surface and secured with screws S. Because the plates LP, MP are on the surface of the bone, they are subject to the large bending moments alluded to previously. As a result, the plates LP, MP are usually quite thick in order to prevent breakage. Because of this bulk, application on a small bone is difficult and it is extremely difficult to bend the plates LP, MP to fit the contour and shape of a particular bone. In addition, thick plates also have the disadvantage of causing significant soft tissue irritation and often require removal.
Because bending forces on these devices are high, plates require bone screws that are large and strong enough to handle the applied loads. However, these larger screw sizes are often too large for the relatively small size of the distal fragments, resulting in problems that include tenuous or failed fixation, iatrogenic fragmentation of the bone fragment through the relatively large hole that is needed for placement of the screw, and irritation of the soft tissues from bulky hardware. Furthermore, fixation with standard plates is completely dependent on the quality of the screw thread purchase in the bone; severe osteoporosis or highly comminuted fractures result in poor thread purchase and significantly increase the risk of failure. Fragments are typically small and often with a large part of the bone surface covered with articular cartilage (plates/screws cannot be applied to the surface of the joint) leaving little to no room for plate application. Plates cannot interfere or cross in the coronoid or olecranon fossa, resulting in further reduction of the area available for plate application.
Plates and screws are subject to large bending moments from cantilever bending as load is applied to the bone. Plates fixed with standard screws are completely dependent on thread purchase in the bone in order to achieve structural rigidity. Unfortunately, often the size and quality of the soft cancellous bone in the supracondylar fragments is insufficient to provide this strength, resulting in screw cut-out, failure, or loss of reduction.
Locking screws (i.e., screws that lock into the plate by threading into the plate) tend to reduce some of the failures related to poor thread purchase. However, since locking screws require a threaded hole in the plate, this design increases the bulk of the plate further. In addition, since locking screws are still subject to the same cantilever bending loads, the use of locking screws does not eliminate the need for relatively large screws for strength. Large screws introduce the related problems of soft tissue irritation, bulky hardware, and iatrogenic fracture from placement of large screw holes in small fragments.
The many variations on basic plate and screw design are a reflection on the multiple attempts to address these issues with supracondylar fracture fixation. Most changes simply involve varying the location of plate application or variation of the shape of the plate to match the surface bone contour. All share the common problem faced by the conflicting need to use a large enough plate to handle the load while avoiding the problems associated with bulk and screw purchase and strength in the distal fragments. In all of these designs, the generation of large cantilever bending loads can create large stresses on both implants and the bone implant interface.
For instance, one known approach is to use a ‘Y’ shaped plate applied to the posterior surface with arms that extend down the medial and lateral column. This plate design is unable to address fixation of very distal articular fragments since screw fixation of such fragments must enter from the non-articular surfaces directly from the medial or lateral side and not posteriorly. Also, these plates are at a mechanical disadvantage and subject to very large bending moments, since the primary arc of motion in flexion and extension occurs in a plane that is perpendicular to the plate surface. Unless the plate is quite thick, it will bend or break.
Another approach is to apply plates on the medial column, the lateral columns, or both, as in FIG. 10. These plates are oriented in a plane that is parallel to the arc of motion. Since these plates are subject to large medial/lateral bending loads, they still need to be thick enough to resist breakage. On the other hand, since they lie directly on the medial and lateral surfaces, they are relatively subcutaneous and prone to cause soft tissue irritation. Another problem with medial/lateral plate application is that the medial and lateral sides of the bone have a complex shape, making it difficult to design and manufacture a plate that matches the complex bone morphology.
Another problem with medial or lateral plates is that they have to be applied over the medial or lateral epicondyle respectively. Unfortunately, these locations are the attachment sites for the strong forearm muscles, requiring the surgeon to detach or release the muscles from bone in order to apply the plate; this can result in tendinitis and loss of muscle strength. It is difficult for these detached tendon groups to heal back to the bone since there is a bulky plate applied to the normal site of attachment. Moreover, the extensive dissection often strips the bone fragment of its only blood supply, resulting in delayed union, non-union, or even bone death (osteonecrosis).
One approach to treating humerus shaft fractures HF is to use an intramedullary nail 40, as shown in FIGS. 11a and 11b. Intramedullary nails 40 can be effective for treatment of shaft fractures of the long bones and are placed through the central canal of the bone and can additionally be secured on the proximal and/or distal sides with interlocking crossing screws 42, as shown in FIG. 11b. Nails have the advantage of a central position in the canal of the bone, aligned with the neutral axis of the bone and better positioned to resist bending loads. Since they reside inside the bone, nails can be relatively bulky yet avoid the issue of soft tissue irritation. Moreover, since the nail achieves some purchase along the entire inner canal, bending forces are distributed over a wider area of the implant, creating a stronger construct. Nails have an additional advantage over plates since they are not as dependent on thread purchase in bone. Unfortunately, these standard nails are not an effective solution for supracondylar fractures of the humerus, since the canal does not extend into the supracondylar region and the nail would obstruct and interfere with the coronoid/olecranon fossas. There is no rigid nail yet designed that extends along the lateral or medial column distally up into the central canal in the shaft.
Another type of nail that has been used for the treatment of supracondylar fractures of the elbow are the so-called flexible nails 44, such as Enders' nails, as shown in FIG. 12 at the site of a humeral diaphysis fracture HDF. These nails 44 have some degree of flexibility and are passed up through a hole in the medial or lateral epicondyles and directed into the canal proximally. These nails are thin enough to have some flex to them to allow them to curve up past the junction between the central canal in the midshaft of the bone and then flare into the medial or lateral epicondyle. However, because these nails 44 are thin enough to be flexible, they do not provide any effective means for rigid fixation proximally, resulting in motion. In addition, these nails 44 do not provide a means for distal fixation of articular fragments. Finally, because there is a limit to the amount of flexibility in these nails, they have been limited to entry sites at the epicondyles and do not extend fixation down to the condyles where it is often needed. For this reason they have been ineffective for these types of fractures.
A variation of Enders' nails uses a clip that could be attached to the distal end of the nail at the entry site and screwed into the adjacent bone. Although this clip and screw help prevent the nail from backing out and rotating, they do not provide resistance to bending moments or fixation of articular fragments.
Finally, another method of treating supracondylar fractures is to use an external fixator as seen at 46 in FIG. 13, either alone or in combination with other methods. External fixation may not completely or adequately reduce the fracture, and often results in prolonged immobilization and significant residual stiffness and dysfunction. Its use is primarily limited to salvage of very difficult cases. Some of the external fixation devices use rods outside the body on either side of the arm to provide paired attachment sites to crossing pins or wires.
Similar problems and fixation challenges occur with periarticular fractures of long bones at other anatomic locations. For example, supracondylar fractures of the femur, fractures of the proximal tibial plateau, and pilon fractures of the lower tibia are other sites subject to similar issues caused by large cantilever bending loads, small periarticular fragments size, poor bone quality, and intimate proximity of adjacent vital soft tissue structures at risk with bulky hardware. These other anatomic locations often present nearly identical problems related to existing methods of fixation.
Implants exist that have a portion extended into an intramedullary canal/cavity on a bone with a fracture. One exemplary construction is shown in U.S. Pat. No. 6,706,046. U.S. Pat. No. 6,706,046 discloses an implant with an intramedullary portion that transitions to an offset extension that is secured to a bone part that is produced by a fracture. In this design, the extension is offset from the long axis of the nail toward the side of entry of screws that penetrate the extension, thereby positioning the extension more superficial than the superficial surface of the nail. This configuration allows a nail to be inserted into a tubular bone while facilitating apposition outside the surface of said tubular bone. As depicted in FIG. 14 of U.S. Pat. No. 6,706,046, the implant must initially be placed at a relatively large angle to allow introduction into the intramedullary cavity/canal. As the implant is advanced into the cavity/canal, it is progressively angularly reoriented to allow the offset extension to seat at the unstable bone fragment for connection thereto. Based upon the depicted geometry, the implant would have to be sufficiently flexible to allow placement in its operative position through the above-mentioned assembly routine. The ability to reconfigure the implant lessens its rigidity and thus its ability to stably maintain a relationship between stable and unstable bone parts that are set, utilizing the implant, preparatory to the healing process. In addition, since the geometry of this design is intended to position the extension out through the side of a tubular bone for fixation along the surface of the tubular bone, it cannot be used for fixation of a terminal fragment that extends beyond the tubular portion of the bone, whether said fragment is either inline with or deep to the longitudinal axis of the intramedullary axis of the tubular bone.
Further, the configuration of the implant makes it impractical for use at many fracture sites.
Implant designers continue to be challenged to make implants with ever greater strength and stability within the geometrical confines of the human body. This is particularly a challenge with implants that reside partially, or fully, within an intramedullary cavity/canal when operatively positioned.
Typically, the intramedullary portion of the implant has strategically located openings to accept fixation components/elements. Jigs/guides are commonly utilized to produce bores in the bone to axially coincide with implant openings that reside within the intramedullary cavity/canal with the implant operatively positioned.
The structural integrity of implants of this type is dictated by the rigidity of the implant itself, the rigidity of the fixation components/elements, and tenacity of the engagement of the fixation components/elements with bone. It is not possible to individually focus on any of these design criteria in attempting performance optimization since these criteria compete with each other.
For example, effective anchoring of the fixation components/elements to the bone generally demands a relatively large diameter, threaded construction to minimize the likelihood of releasing of the fixation components/elements from the bone or bending of the fixation components/elements. Each fixation component/element demands the same diameter opening in the intramedullary portion of the implant. These implant openings potentially weaken the intramedullary portion of the implant.
Designers are thus left with the options of either contending with a weakened implant or increasing the dimensions of the intramedullary portion of the implant to accommodate more robust fixation components/elements. The former option has potentially dangerous consequences. The latter option may produce a construction that is impractical or difficult to use.
The medical profession has generally contended with, and continues to contend with, these problems since no viable solution has been developed to date.